The present invention relates to a method for the continuous preparation of a powdered product from a product in the liquid state in the form of a solution-, suspension- or emulsion-type mixture in an aqueous medium.
The invention relates, in particular, to a method for the continuous preparation of powdered milk from liquid milk and to a powdered milk obtained by such a method.
Conventionally, powdered milk is obtained industrially by carrying out various operations for obtaining, from liquid milk generally having a dry matter concentration of approximately 12%, a powdered milk having a dry matter concentration of approximately 96 to 98% without adversely affecting the nutritional and functional properties of the milk. Owing to the heat sensitivity of milk, the operation necessitates special drying conditions which are, in particular, the application of a low temperature during the liquid milk evaporation and concentration operations and recourse to short residence times during the passage from the liquid state to the solid state.
In addition, the resultant powdered milk has to meet strict criteria in terms of functional and usage properties such as density, porosity, wettability, complete and instantaneous rehydratability.
Up until now, the drying of milk has involved a plurality of successive operations, namely:                vacuum evaporation of the milk in the liquid state having an initial dry matter concentration of approximately 12% in order to obtain a dry matter concentration of between 50 and 60%;        spray drying of the milk issuing from the preceding operation and in which the milk is atomised into very fine droplets which are placed in direct contact with hot air. This operation produces powdered milk of which the dry matter content is approximately 92%;        final drying in a fluidised bed to achieve the final dry matter content of approximately 96 to 98%; and        milling and sieving to calibrate the milk powder prior to the packaging thereof        
This method of producing powdered milk, used up until now, has drawbacks.
Spray drying is expensive, in particular, in terms of investment and power consumption. The power consumption is approximately 2.6 to 2.8 kg of steam and 0.8 kW per kg of evaporated water, making this operation by far the greatest power consumer of the entire method, with approximately 70% of the thermal power and approximately 40% of the electrical power.
In addition, even if spray drying can be applied to differing milk compositions, including powdered milk substitutes, this method does not permit the treatment of liquid milk which is enriched with vaporisable ingredients such as flavourings, for example, which would be eliminated during drying, so the development and production of new products such as vitaminised milk or flavoured milk, for example, by spray drying cannot be considered.
Moreover, the document U.S. Pat. No. 5,596,815 which constitutes the closest prior art discloses a process for drying raw material having a substantial proportion of moisture and in which the raw material is transferred into a mixer with a predetermined proportion of a relatively dry material. The material is then transferred into a pelletiser, then a drier. A proportion of the material obtained is transferred into a cooler and into a mill, and the milled material is recycled into the mixer.
However, the main drawback of this method is that the resultant product does not have the required functional and usage properties and does not achieve the qualities of the products obtained by conventional methods as it has a high density and excessively low porosity and solubility.
It is accordingly the object of the invention to propose a method for the continuous production of a powdered product from a product in the liquid state which enables the aforementioned drawbacks to be avoided.